Pulse width modulated (PWM) signals are commonly used in electrical and electronic systems. By changing (e.g., modulating) the duty cycles of, e.g., a sequence of periodic square waves of the PWM signal, information can be carried in the varying duty cycles of the waveforms. PWM signals may be used to carrier control signals or digital information (e.g., speed/audio information) in a wide variety of systems, such as control systems, power systems, audio systems, or the like. The waveforms in a PWM signal may also be referred to as square wavers or pulses herein, and the PWM signal is said to comprise a plurality of pulses (or a plurality of square waves).
In some applications, such as high frequency class-D amplifier applications, the PWM signals generated by the PWM modulator may have pulses with duty cycles outside a pre-determined range, wherein the pre-determined range may be, e.g., between about 2% and about 98%. For example, some pulses may have duty cycles that are too high (e.g., larger than about 98%) or too low (e.g., less than 2%). Pulses with duty cycles outside the pre-determined range may cause problem for subsequent processing in the system. For example, pulses with too low of a duty cycle may fail to set or reset registers properly in a digital system, which may result in an error in the output level of the system. Therefore, it may be advantageous to filter the pulses of the PWM signal to remove pulses having duty cycles outside the pre-determined range.
Challenges exist in the design of a filtering circuit for the PWM signal. For example, a simple RC filter may not be able to remove all pulses in the PWM signal that have duty cycles outside the pre-determined range. In addition, the RC filter may introduce distortions into the filtered pulses, thereby distorting the shape of the pulses (e.g., changing the width of the pulse). For applications such as high frequency class-D amplifier applications, distortion in the filtered pulses of the PWM signal may seriously degrade the system performance. Therefore, there is a need in the art for a filtering circuit that can reliably remove pulses of a PWM signal that have duty cycles outside a pre-determined range while introducing little or no distortion to the filtered pulses.